Method of making a microporous material

ABSTRACT

A method for producing a microporous material comprising the steps of: providing an ultrahigh molecular weight polyethylene (UHMWPE); providing a filler; providing a processing plasticizer; adding the filler to the UHMWPE in a mixture being in the range of from about 1:9 to about 15:1 filler to UHMWPE by weight; adding the processing plasticizer to the mixture; extruding the mixture to form a sheet from the mixture; calendering the sheet; extracting the processing plasticizer from the sheet to produce a matrix comprising UHMWPE and the filler distributed throughout the matrix; stretching the microporous material in at least one direction to a stretch ratio of at least about 1.5 to produce a stretched microporous matrix; and subsequently calendering the stretched microporous matrix to produce a microporous material which exhibits improved physical and dimensional stability properties over the stretched microporous matrix.

RELATED APPLICATIONS

This application is a divisional application based upon U.S. patentapplication Ser. No. 15/345,184 filed Nov. 7, 2016, now abandoned, whichwas a continuation of U.S. application Ser. No. 11/854,580 filed Sep.13, 2007, now abandoned, which was a division of U.S. application Ser.No. 11/006,333 filed Dec. 7, 2004, now U.S. Pat. No. 7,445,735.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present application discloses a microporous membrane and a methodfor making the same.

2. Description of the Prior Art

A microporous membrane comprising a very high molecular weightpolyolefin and an inert filler material was taught by Larsen, U.S. Pat.No. 3,351,495. The general principles and procedures of U.S. Pat. No.3,351,495 are incorporated herein by reference.

Kono et al., U.S. Pat. No. 4,600,633 teaches a polyethylene superthinfilm and a process for the production of the same. In this process anultra high molecular weight polyethylene (herein after UHMWPE) isdissolved in a solvent then extruded to form a gel sheet. The gel sheetthen undergoes a first extraction step to remove the solvent. After thefirst extraction, the sheet is heated and stretched. The stretched sheetthen undergoes a second extraction step to remove solvent. The resultingproduct then undergoes a compression treatment at a temperature of 80°to 140° centigrade. This reference does not use a filler in its UHMWPE.The gel sheet is not calendered prior to solvent extraction. Theresulting product is a thin film with a tensile modulus of at least 2000kg/cm² a breaking strength of at least 500 kg/cm² and which, issubstantially free from pores.

Schwarz et al., U.S. Pat. No. 4,833,172 teaches a stretched microporousmaterial. In this process an UHMWPE and a siliceous filler are dissolvedin a plasticizer then extruded to form a gel sheet. In this process thegel sheet may optionally be calendered prior to a solvent extraction.The gel sheet then undergoes a solvent extraction to remove theplasticizer. After the extraction, the sheet is then stretched.

SUMMARY OF THE INVENTION

A method for producing a microporous material comprising the steps of:providing an ultrahigh molecular weight polyethylene (herein afterUHMWPE); providing a particulate filler; and providing a processingplasticizer where the processing plasticizer is typically a liquid atroom temperature. The filler and the UHMWPE and the processingplasticizer are mixed, the resulting mixture may comprise from about 1:9to about 15:1 filler to UHMWPE by weight. The resulting mixture is thenextruded and immediately processed (either calendered, blown or cast) toform a sheet. The formed sheet is then subjected to an extraction step,where the plasticizer is partially (or fully) removed. The resultingsheet is a matrix which comprises UHMWPE, oil (if not fully extracted),and the filler. The extraction step has rendered this matrixmicroporous. The filler is distributed throughout this microporousmatrix, with the filler constituting from 5 percent to 95 percent byweight of the microporous matrix. The microporous matrix has a networkof interconnecting pores communicating throughout the microporousmatrix. The pores constitute from 25 percent to 90 percent by volume ofthe microporous matrix. Then, this microporous matrix is stretched. Thisproduces a stretched microporous matrix that is not very dimensionallystable at elevated temperature. The stretched microporous matrix is thencalendered to produce a final microporous material with much improveddimensional stability and physical properties.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawings information about some of the embodiments of the invention; itbeing understood, however, that this invention is not limited to theprecise information shown.

FIG. 1 is a graph of data obtained using a mercury Porosimeter whichplots pore diameter to pore volume for a microporous matrix beforestretch made in accordance with the teachings of the prior art;

FIG. 2 is a graph of data obtained using a mercury Porosimeter whichplots pore diameter to pore volume for a material made in accordancewith the teachings of the prior art, where stretch is uniaxial in themachine direction;

FIG. 3 is a graph of data obtained using a mercury Porosimeter whichplots pore diameter to pore volume for a material made in accordancewith the teachings of the prior art, where stretch is a biaxial stretch;

FIG. 4 is a graph of data obtained using a mercury Porosimeter whichplots pore diameter to pore volume for a membrane made by the inventedprocess, where post stretch calendering is performed at moderatepressure;

FIG. 5 is a graph of data obtained using a mercury Porosimeter whichplots pore diameter to pore volume for the membrane made by the inventedprocess, where post stretch calendering is performed at a highercompression pressure.

DETAILED DESCRIPTION OF THE INVENTION

A method for producing a microporous material comprising the steps of:providing an ultrahigh molecular weight polyethylene (herein afterUHMWPE); providing a particulate filler; and providing a processingplasticizer where the processing plasticizer is a liquid at roomtemperature. The UHMWPE, filler and plasticizer are all described ingreater detail below. The UHMWPE, filler and plasticizer are mixedtogether to form a mixture. The mixture is extruded through a die (e.g.slot die or blown film die) to form a sheet. The sheet may be furtherprocessed, by casting onto a chilled roller, or calendered, or blown.The cast or calendered sheet is then subjected to an extraction step topartially (or fully) remove the plasticizer and forms thereby amicroporous matrix. The matrix comprises UHMWPE, plasticizer if notfully extracted, and the particulate filler distributed throughout thematrix. The filler constitutes from 5 percent to 95 percent by weight ofthe microporous matrix. The microporous matrix has a network ofinterconnecting pores communicating throughout the microporous matrix.The pores constitute from 25 percent to 90 percent by volume of themicroporous matrix. The microporous matrix is stretched. The stretchingprocess is described in greater detail below. The stretched microporousmatrix is not dimensionally stable at elevated temperatures. Thestretched microporous matrix is subsequently calendered to produce thefinal microporous material that is dimensionally stable even at elevatedtemperatures.

Ultrahigh molecular weight polyethylene (UHMWPE) can be defined as apolyethylene having an intrinsic viscosity of least about 18deciliters/gram. In many cases the intrinsic viscosity is at least about19 deciliters/gram. Although there is no particular restriction on theupper limit of the intrinsic viscosity, the intrinsic viscosity isfrequently in the range of from about 18 to about 39 deciliters/gram. Anintrinsic viscosity in the range of from about 18 to about 32deciliters/gram is most common.

As used herein and in the claims, intrinsic viscosity is determined byextrapolating to zero concentration the reduced viscosities or theinherent viscosities of several dilute solutions of the UHMWPE where thesolvent is freshly distilled decahydronaphthalene to which 0.2 percentby weight, 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid,neopentanetetrayl ester [CAS Registry No. 6683-19-8] has been added. Thereduced viscosities or the intrinsic viscosities of the UHMWPE areascertained from relative viscosities obtained at 135° C. using anUbbelohde No. 1 viscometer in accordance with the general procedures ofASTM D 4020-81, except that several dilute solutions of differingconcentration are employed. ASTM D 4020-81 is, in its entirety,incorporated herein by reference.

Sufficient UHMWPE should be present in the matrix to provide itsproperties to the microporous material. Other thermoplastic organicpolymers may also be present in the matrix, so long as their presencedoes not materially affect the properties of the microporous material inan adverse manner. The amount of the other thermoplastic polymers whichmay be present depends upon the nature of such polymers. In general, agreater amount of other thermoplastic organic polymer may be used if themolecular structure contains little branching, few long sidechains, andfew bulky side groups, than when there is a large amount of branching,many long sidechains, or many bulky side groups. For this reason, theexemplary thermoplastic organic polymers that may be mixed with theUHMWPE are low density polyethylene, high density polyethylene,poly(tetrafluoroethylene), polypropylene, copolymers of ethylene, suchas ethylene-butene or ethylene-hexene, copolymers of propylene,copolymers of ethylene and acrylic acid, and copolymers of ethylene andmethacrylic acid. If desired, all or a portion of the carboxyl groups ofcarboxyl-containing copolymers may be neutralized with sodium, zinc orthe like. Usually at least about 70 percent UHMWPE (or 70 percent UHMWPEand other thermoplastic organic polymers), based on the weight of thematrix, will provide the desired properties to the microporous material.

The particulate filler may be in the form of ultimate particles,aggregates of ultimate particles, or a combination of both. In mostcases, at least about 90 percent by weight of the filler has grossparticle sizes in the range of from about 5 to about 40 micrometers. Ifthe filler used is titanium dioxide (TiO2) the gross particle size canrange from 0.005 to 45 micrometers. In another embodiment using titaniumdioxide (TiO2) as a filler, the gross particle size ranges from 0.1 to 5micrometers. In another case, at least about 90 percent by weight of thefiller has a gross particle size in the range of from about 10 to about30 micrometers. It is expected that filler agglomerates will be reducedin size during processing of the ingredients. Accordingly, thedistribution of gross particle sizes in the microporous material may besmaller than in the raw filler itself. Particle size is determined byuse of a Model TAII Coulter counter (Coulter Electronics, Inc.)according to ASTM C 690-80, but modified by stirring the filler for 10minutes in Isoton II electrolyte (Curtin Matheson Scientific, Inc.)using a four-blade, 4.445 centimeter diameter propeller stirrer. ASTM C690-80 is, in its entirety, incorporated herein by reference.

The particulate filler will on average have an ultimate particle size(irrespective of whether or not the ultimate particles are agglomerated)which is less than about 30 micrometer as determined by transmissionelectron microscopy. Often the average ultimate particle size is lessthan about 0.05 micrometer. In one embodiment the average ultimateparticle size of filler is approximately 20 micrometers (when aprecipitated silica is used).

Use of fillers in a polymer matrix is well documented. In generalexamples of suitable fillers include siliceous fillers, such as: silica,mica, montmorillonite, kaolinite, asbestos, talc, diatomaceous earth,vermiculite, natural and synthetic zeolites, cement, calcium silicate,clay, aluminum silicate, sodium aluminum silicate, aluminumpolysilicate, alumina silica gels, and glass particles. In addition tothe siliceous fillers other particulate substantially water-insolublefillers may also be employed. Examples of such optional fillers includecarbon black, activated carbon, carbon fibers, charcoal, graphite,titanium oxide, iron oxide, copper oxide, zinc oxide, lead oxide,tungsten, antimony oxide, zirconia, magnesia, alumina, molybdenumdisulfide, zinc sulfide, barium sulfate, strontium sulfate, calciumcarbonate, and magnesium carbonate.

Silica and the clays are the most useful siliceous fillers. Of thesilicas, precipitated silica, silica gel, or fumed silica is most oftenused.

The particulate filler which has been found to work well is precipitatedsilica. It is important to distinguish precipitated silica from silicagel, inasmuch as these different materials have different properties.Reference in this regard is made to R. K. Iler, The Chemistry of Silica,John Wiley & Sons, New York (1979), Library of Congress Catalog No. QD181.S6144, the entire disclosure of which is incorporated herein byreference. Note especially pages 15-29, 172-176, 218-233, 364-365,462-465, 554-564, and 578-579. Silica gel is usually producedcommercially at low pH by acidifying an aqueous solution of a solublemetal silicate, typically sodium silicate, with acid. The acid employedis generally a strong mineral acid such as sulfuric acid or hydrochloricacid although carbon dioxide is sometimes used. Inasmuch as there isessentially no difference in density between gel phase and thesurrounding liquid phase while the viscosity is low, the gel phase doesnot settle out, that is to say, it does not precipitate. Silica gel,then, may be described as a non-precipitated, coherent, rigid,three-dimensional network of contiguous particles of colloidal amorphoussilica. The state of subdivision ranges from large, solid masses tosubmicroscopic particles, and the degree of hydration from almostanhydrous silica to soft gelatinous masses containing on the order of100 parts of water per part of silica by weight, although the highlyhydrated forms are only rarely used in the present invention.

Precipitated silica on the other hand, is usually produced commerciallyby combining an aqueous solution of a soluble metal silicate, ordinarilyalkali metal silicate such as sodium silicate, and an acid so thatcolloidal particles will grow in weakly alkaline solution and becoagulated by the alkali metal ions of the resulting soluble alkalimetal salt. Various acids may be used, including the mineral acids, butthe preferred material is carbon dioxide. In the absence of a coagulant,silica is not precipitated from solution at any pH. The coagulant usedto effect precipitation may be the soluble alkali metal salt producedduring formation of the colloidal silica particles, it may be addedelectrolyte such as a soluble inorganic or organic salt, or it may be acombination of both. Precipitated silica, then, may be described asprecipitated aggregates of ultimate particles of colloidal amorphoussilica that have not at any point existed as macroscopic gel during thepreparation. The sizes of the aggregates and the degree of hydration mayvary widely.

Precipitated silica powders differ from silica gels in that they havebeen pulverized in ordinarily having a more open structure, that is, ahigher specific pore volume. However, the specific surface area ofprecipitated silica as measured by the Brunauer, Emmett, Teller (BET)method using nitrogen as the adsorbate, is often lower than that ofsilica gel.

Many different precipitated silicas may be employed in the presentinvention, but the preferred precipitated silicas are those obtained byprecipitation from an aqueous solution of sodium silicate using asuitable acid such as sulfuric acid or hydrochloric acid. Carbon dioxidecan also be used to precipitate the silica. Such precipitated silicasare known and processes for producing them are described in detail inU.S. Pat. No. 2,940,830, the entire disclosure of which is incorporatedherein by reference, including the processes for making precipitatedsilicas and the properties of the products.

In the proceeding process for producing a microporous matrix, extrusionand calendering are facilitated when the substantially water-insolublefiller carries much of the processing plasticizer. The capacity of thefiller particles to absorb and hold the processing plasticizer is afunction of the surface area of the filler. It is therefore preferredthat the filler have a high surface area. High surface area fillers arematerials of very small particle size, materials having a high degree ofporosity or materials exhibiting both characteristics. Usually thesurface area of the filler itself is in the range of from about 20 toabout 400 square meters per gram as determined by the Brunauer, Emmett,Teller (BET) method according to ASTM C 819-77 using nitrogen as theadsorbate but modified by outgassing the system and the sample for onehour at 130° C. Preferably the surface area is in the range of fromabout 25 to about 350 square meters per gram. ASTM C 819-77 is, in itsentirety, incorporated herein by reference. Inasmuch as it is desirableto essentially retain the filler in the microporous matrix sheet, it ispreferred that the substantially water-insoluble filler be substantiallyinsoluble in the processing plasticizer and substantially insoluble inthe organic extraction liquid when microporous matrix sheet is producedby the above process.

The processing plasticizer is typically a liquid at room temperature andusually it is processing oil such as paraffinic oil, naphthenic oil, oran aromatic oil. Suitable processing oils include those meeting therequirements of ASTM D 2226-82, Types 103 and 104. It has been foundthat oils which have a pour point of less than 22° C. according to ASTMD 97-66 (reapproved 1978) work well. Oils having a pour point of lessthan 10° C. also work well. Examples of suitable oils include, but arenot limited to, Shellflex® 412 and Shellflex® 371 oil (Shell Oil Co.)which are solvent refined and hydrotreated oils derived from naphtheniccrude. ASTM D 2226-82 and ASTM D 97-66 (reapproved 1978) are, in theentireties, incorporated herein by reference. It is expected that othermaterials, including the phthalate ester plasticizers such as dibutylphthalate, bis (2-ethylhexyl) phthalate, diisodecyl phthalate,dicyclohexyl phthalate, butyl benzyl phthalate, ditridecyl phthalate andwaxes, will function satisfactorily as processing plasticizers. Theprocessing plasticizer has little solvating effect on the thermoplasticorganic polymer at 60° C., only a moderate solvating effect at elevatedtemperatures on the order of about 100° C., and a significant solvatingeffect at elevated temperatures on the order of about 200° C.

Minor amounts, usually less than about 5 percent by weight, of othermaterials used in processing such as lubricant, organic extractionliquid, surfactant, water, and the like, may optionally also be present.Yet other materials introduced for particular purposes may optionally bepresent in the microporous material in small amounts, usually less thanabout 15 percent by weight. Examples of such materials includeantioxidants, ultraviolet light absorbers, flame retardants, reinforcingfibers such as carbon fiber or chopped glass fiber strand, dyes,pigments, and the like. The balance of the microporous material,exclusive of filler and any impregnate applied for one or more specialpurposes, is essentially the thermoplastic organic polymer andplasticizer (if not fully extracted).

Then the filler, thermoplastic organic polymer powder, processingplasticizer and other additives are mixed until a substantially uniformmixture is obtained. This uniform mixture may also contain otheradditives such as minor amounts of lubricant and antioxidant. The weightratio of filler to polymer powder employed in forming the mixture isessentially the same as that of the stretched microporous material to beproduced. The ratio of filler to UHMWPE in this mixture is in the rangeof from about 1:9 to about 15:1 filler to UHMWPE by weight. Theparticulate filler constitutes from about 5 percent to about 95 percentby weight of that microporous material. Frequently, such fillerconstitutes from about 45 percent to about 90 percent by weight of themicroporous material. From about 55 percent to about 80 percent byweight is used in one of the embodiments of the invention. The ratio ofthe UHMWPE to the processing plasticizer is 1:30 to 3:2 by weight. Ratioof filler to processing plasticizer is 1:15 to 3:1 by weight.

In the extrusion and calendering process, the mixture, together withadditional processing plasticizer, is introduced to the heated barrel ofa screw extruder. Attached to the extruder is a sheeting die. Acontinuous sheet formed by the die is forwarded without drawing to apair of heated calender rolls acting cooperatively to form continuoussheet of lesser thickness than the continuous sheet exiting from thedie.

The continuous sheet is subjected to an extraction step where processingplasticizer is partially or fully removed there from. The extractionstep may include one or more steps. For example, the continuous sheetfrom the calender then passes to a first extraction zone where theprocessing plasticizer is substantially removed by extraction with anorganic liquid which is a good solvent for the processing plasticizer, apoor solvent for the organic polymer, and more volatile than theprocessing plasticizer. Usually, but not necessarily, both theprocessing plasticizer and the organic extraction liquid aresubstantially immiscible with water. There are many organic extractionliquids that can be used. Examples of suitable organic extractionliquids include but are not limited to hexane, alkanes of varying chainlengths, 1,1,2-trichloroethylene, perchloroethylene, 1,2-dichioroethane,1,1,1-trichloroethane, 1,1,2-trichloroethane, methylene chloride,chloroform, isopropyl alcohol, diethyl ether and acetone. The continuoussheet then passes to a second extraction zone where the residual organicextraction liquid is substantially removed by: heat, steam and/or water.The continuous sheet is then passed through forced air dryer forsubstantial removal of residual water and remaining residual organicextraction liquid. From the dryer the continuous sheet, which is amicroporous matrix, is passed to a take-up roll.

The microporous matrix comprises a filler, UHMWPE, and optionalmaterials in essentially the same weight proportions as those discussedabove in respect of the stretched microporous matrix. The matrix mightalso have some plasticizer if it is not fully extracted. The residualprocessing plasticizer content is usually less than 20 percent by weightof the microporous matrix and this may be reduced even further byadditional extractions using the same or a different organic extractionliquid.

In the microporous matrix, the pores constitute from about 25 to about90 percent by volume. In many cases, the pores constitute from about 30to about 80 percent by volume of the microporous matrix. One of theembodiments has from 50 to 75 percent of the volume of the microporousmatrix is pores. The porosity of the microporous matrix, expressed aspercent by volume. Unless impregnated, the porosity of the stretchedmicroporous matrix is greater than the porosity of the microporousmatrix before stretching.

As used herein and in the claims, the porosity (also known as voidvolume) of the microporous material, expressed as percent by volume, isdetermined according to the equation:Porosity=100[1−d ₁ /d ₂]where d₁ is the density of the sample which is determined from thesample weight and the sample volume as ascertained from measurements ofthe sample dimensions and d₂ is the density of the solid portion of thesample which is determined from the sample weight and the volume of thesolid portion of the sample. The volume of the solid portion of thesample can be determined using a Quantachrome stereopycnometer(Quantachrome Corp.) in accordance with the accompanying operatingmanual.

The volume average diameter of the pores of the microporous sheet can bedetermined by mercury porosimetry using an Autoscan mercury porosimeter(Quantachrome Corp.). Mercury Intrusion/Extrusion is based on forcingmercury (a non-wetting liquid) into a porous structure under tightlycontrolled pressures. Since mercury does not wet most substances andwill not spontaneously penetrate pores by capillary action, it must beforced into the voids of the sample by applying external pressure. Thepressure required to fill the voids is inversely proportional to thesize of the pores. Only a small amount of force or pressure is requiredto fill large voids, whereas much greater pressure is required to fillvoids of very small pores.

In operating the porosimeter, a scan is made in the high pressure range(from about 138 kilopascals absolute to about 227 megapascals absolute).If about 2 percent or less of the total intruded volume occurs at thelow end (from about 138 to about 250 kilopascals absolute) of the highpressure range, the volume average pore diameter is taken as twice thevolume average pore radius determined by the porosimeter. Otherwise, anadditional scan is made in the low pressure range (from about 7 to about165 kilopascals absolute) and the volume average pore diameter iscalculated according to the equation:

$d = {{2\lbrack {\frac{v_{1}r_{1}}{w_{1}} + \frac{v_{2}r_{2}}{w_{2}}} \rbrack}/\lbrack {\frac{v_{1}}{w_{1}} + \frac{v_{2}}{w_{2}}} \rbrack}$where d is the volume average pore diameter, v₁ is the total volume ofmercury intruded in the high pressure range, v₂ is the total volume ofmercury intruded in the low pressure range, r₁ is the volume averagepore radius determined from the high pressure scan, r₂ is the volumeaverage pore radius determined from the low pressure scan, w₁ is theweight of the sample subjected to the high pressure scan, and w₂ is theweight of the sample subjected to the low pressure scan.

Large pores require lower pressures for the mercury to intrude into thepore volume while smaller pores require the higher pressures forintrusion into the pore volumes. In FIG. 1 , approximately 20% of thepores are smaller than 0.02 micrometers. DV/log d represents the changein pore volume with the change in log of pore diameter. Thus as can beseen from FIG. 1 , there are a large number of pores that have adiameter of approximately 0.016 micrometers, and the peak height isseveral magnitudes higher than any other peak. In this representation,the peak areas and heights represent the relative number of pores at thecorresponding log of pore diameter.

The volume average diameter of the pores of the precursor microporousmatrix is usually a distribution from about 0.01 to about 1.0micrometers, as seen in FIG. 1 . By stretching the precursor materialone can obtain pores which are greater than 1 micrometer in size. Theresulting pore distribution of this stretched material can be seen inFIGS. 2 and 3 . Depending on the amount of stretch it is possible toobtain pores greater than 20 to 30 micrometers. Then through thesubsequent calendering step the pore size can be selectively reducedfrom the enlarged pore distribution. One example at this modifieddistribution of the average diameter of the pores is in the range offrom about 0.01 to about 0.8 micrometers for the resulting microporousmaterial which is stretched and calendered. In another embodiment theresulting microporous material has a distribution of average diameter ofthe pores of from about 0.01 to about 0.6 micrometers, as seen in FIG. 4. The volume average diameter of the pores of the microporous matrix isdetermined by the mercury porosimetry method.

The stretched microporous matrix may be produced by stretching themicroporous matrix in at least one stretching direction to a stretchratio of at least about 1.5. In many cases, the stretch ratio is atleast about 1.7. In another embodiment it is at least about 2.Frequently, the stretch ratio is in the range of from about 1.5 to about15. Often the stretch ratio is in the range of from about 1.7 to about10. In another embodiment, the stretch ratio is in the range of fromabout 2 to about 6. As used herein and in the claims, the stretch ratiois determined by the formula:S=L ₂ /L ₁where S is the stretch ratio, L₁ is the distance between two referencepoints located on the microporous matrix and on a line parallel to thestretching direction, and L₂ is the distance between the same tworeference points located on the stretched microporous material. When thestretching is done in two directions, the stretching in the twodirections may be performed either sequentially or simultaneously.

The temperatures at which stretching is accomplished may vary widely.Stretching may be accomplished at about ambient room temperature, butusually elevated temperatures are employed. The microporous matrix maybe heated by any of a wide variety of techniques prior to, during,and/or after stretching. Examples of these techniques include radiativeheating such as that provided by electrically heated or gas firedinfrared heaters, convective heating such as that provided byrecirculating hot air, and conductive heating such as that provided bycontact with heated rolls. The temperatures which are measured fortemperature control purposes may vary according to the apparatus usedand personal preference. For example, temperature-measuring devices maybe placed to ascertain the temperatures of the surfaces of infraredheaters, the interiors of infrared heaters, the air temperatures ofpoints between the infrared heaters and the microporous matrix, thetemperatures of circulating hot air at points within the apparatus, thetemperature of hot air entering or leaving the apparatus, thetemperatures of the surfaces of rolls used in the stretching process,the temperature of heat transfer fluid entering or leaving such rolls,or film surface temperatures. In general, the temperature ortemperatures are controlled such that the microporous matrix isstretched about evenly so that the variations, if any, in film thicknessof the stretched microporous matrix are within acceptable limits and sothat the amount of stretched microporous matrix outside of those limitsis acceptably low. It will be apparent that the temperatures used forcontrol purposes may or may not be close to those of the microporousmatrix itself since they depend upon the nature of the apparatus used,the locations of the temperature-measuring devices, and the identitiesof the substances or objects whose temperatures are being measured.

In view of the locations of the heating devices and the line speedsusually employed during stretching, gradients of varying temperaturesmay or may not be present through the thickness of the microporousmatrix. Also because of such line speeds, it is impracticable to measurethese temperature gradients. The presence of gradients of varyingtemperatures, when they occur, makes it unreasonable to refer to asingular film temperature. Accordingly, film surface temperatures, whichcan be measured, are best used for characterizing the thermal conditionof the microporous matrix. These are ordinarily about the same acrossthe width of the microporous matrix during stretching although they maybe intentionally varied, as for example, to compensate for microporousmatrix having a wedge-shaped cross-section across the sheet. Filmsurface temperatures along the length of the sheet may be about the sameor they may be different during stretching.

The film surface temperature at which stretching is accomplished mayvary widely, but in general they are such that the microporous matrix isstretched about evenly, as explained above. In most cases, the filmsurface temperatures during stretching are in the range of from about20° C. to about 220° C. Often such temperatures are in the range of fromabout 50° C. to about 200° C. From about 75° C. to about 180° C. isanother range in this embodiment.

Stretching may be accomplished in a single step or a plurality of stepsas desired. For example, when the microporous matrix is to be stretchedin a single direction (uniaxial stretching), the stretching may beaccomplished by a single stretching step or a sequence of stretchingsteps until the desired final stretch ratio is attained. Similarly, whenthe microporous matrix is to be stretched in two directions (biaxialstretching), the stretching can be conducted by a single biaxialstretching step or a sequence of biaxial stretching steps until thedesired final stretch ratios are attained. Biaxial stretching may alsobe accomplished by a sequence of one or more uniaxial stretching stepsin one direction and one or more uniaxial stretching steps in anotherdirection. Biaxial stretching steps where the microporous matrix isstretched simultaneously in two directions and uniaxial stretching stepsmay be conducted in sequence in any order. Stretching in more than twodirections is within contemplation. It may be seen that the variouspermutations of steps are quite numerous. Other steps, such as cooling,heating, sintering, annealing, reeling, unreeling, and the like, mayoptionally be included in the overall process as desired.

Various types of stretching apparatus are well known and may be used toaccomplish stretching of the microporous matrix according to the presentinvention. Uniaxial stretching is usually accomplished by stretchingbetween two rollers wherein the second or downstream roller rotates at agreater peripheral speed than the first or upstream roller. Uniaxialstretching can also be accomplished on a standard tentering machine.Biaxial stretching may be accomplished by simultaneously stretching intwo different directions on a tentering machine. More commonly, however,biaxial stretching is accomplished by first uniaxially stretchingbetween two differentially rotating rollers as described above, followedby either uniaxially stretching in a different direction using a tentermachine or by biaxially stretching using a tenter machine. The mostcommon type of biaxial stretching is where the two stretching directionsare approximately at right angles to each other. In most cases wherecontinuous sheet is being stretched, one stretching direction is atleast approximately parallel to the long axis of the sheet (machinedirection) and the other stretching direction is at least approximatelyperpendicular to the machine direction and is in the plane of the sheet(transverse direction).

After the microporous matrix has been stretched either uniaxially orbiaxially then the stretched microporous matrix is again calendered. Thestretched microporous matrix is forwarded to a pair of heated calenderrolls acting cooperatively to form a membrane of lesser thickness thanthe microporous matrix exiting from the stretching apparatus. Byregulating the pressure exerted by these calender rolls along with thetemperature, the pore size of the final membrane can be controlled asdesired. This allows the manufacturer to adjust the average pore sizewith a degree of control which heretofore has not been seen. The finalpore size will affect other properties such as the Gurley value of themembrane, as well as, improving the dimensional stability of themembrane at temperatures above room temperature of 20° to 25°centigrade.

The figures provided are plots of data collected from mercuryporosimetry. FIG. 1 is a graph showing pore diameter in micrometers forthe precursor membrane extruded through a slot die and calendered, andthen partially extracted of plasticizer. The resulting microporousmatrix has not been stretched or subsequently calendered. FIG. 2 is agraph showing pore diameter in micrometers for a membrane stretcheduniaxially 400% in the machine direction. FIG. 3 is a graph showing porediameter in micrometers for a membrane biaxially stretched. FIG. 4 is agraph showing pore diameter in micrometers for a membrane biaxiallystretched and subsequently calendered through a gap of 25 micrometers.FIG. 5 is a graph showing pore diameter in micrometers for a membranebiaxially stretched and subsequently calendered at a high compressionpressure through a minimal gap. These figures show that compressionsubstantially changes the pore size distribution which is present in thematerial. Also, it is possible to adjust the pore size distribution byadjusting the compression conditions.

The final membrane is the result of stretching a precursor material andsubsequently compressing it to have at least a 5% reduction in thicknessof the stretched precursor material, which is defined above as themicroporous matrix. This microporous material consists essentially of(or comprises): an ultrahigh molecular weight polyethylene (UHMWPE) anda particulate filler distributed throughout the microporous material,where the filler constitutes from about 5 percent to 95 percent byweight of the microporous material. The microporous material has anetwork of interconnecting pores communicating throughout themicroporous material, the pores constituting at least 25 percent byvolume of the microporous material. The microporous material has atensile strength in the machine direction (MD) of greater than 20 N/mm²;the microporous material also has a wet out time of less than 180seconds when silica is used as the filler. It has been observed thatthis microporous material has an electrical resistance of less than 130mohm/mm².

A microporous material where the microporous material consistsessentially of: (or comprises) an ultrahigh molecular weightpolyethylene (UHMWPE) and a particulate filler distributed throughoutthe microporous material, where the filler constitutes from about 5percent to 95 percent by weight of the microporous material. Themicroporous material has a network of interconnecting porescommunicating throughout the microporous material, with the poresconstituting at least 25 percent by volume of the microporous material.This microporous material has no pores greater in size than 1.0micrometers; and where change in volume divided by log d for the poresof this microporous material is less than 2 cc/g.

The resulting microporous material which has been both stretched andcalendered exhibits shrink in the machine direction of less than 10% andhas tensile strength of greater than 25 N/mm² in the machine direction(MD).

The microporous material described above can also include a secondpolymer. The UHMWPE is mixed with a high density (HD) polyethylene toproduce a polyolefin mixture, where the polyolefin mixture has at least50% UHMWPE by weight. The filler used with this polyolefin mixture is ina range of from about 1:9 to about 15:1 filler to polyolefin mixture byweight. The resulting matrix consists essentially of (or comprises)UHMWPE and HD polyethylene and the particulate filler distributedthroughout the matrix. This microporous material has a machine direction(MD) tensile strength of greater than 25 N/mm².

With higher compression after the stretch the resulting microporousmaterial consists essentially of: (or comprises) an ultrahigh molecularweight polyethylene (UHMWPE) and a particulate filler distributedthroughout the microporous material, where the filler constitutes fromabout 5 percent to 95 percent by weight of the microporous material. Themicroporous material has a network of interconnecting porescommunicating throughout the microporous material, with the poresconstituting at least 25 percent by volume of the microporous material.Since the compression pressure determines the resulting pore sizedistribution, the pore structure is highly adjustable. For instance,this microporous material in FIG. 4 has no pores greater in size than0.50 micrometers. The median pore size is between or equal to 0.01 and0.3 micrometers and the pores vary in size by plus or minus 0.2micrometers.

The resulting microporous material which has been both stretched andcalendered exhibits shrink in the machine direction of less than 10% andhas tensile strength of greater than 25 N/mm² in the machine direction(MD).

The microporous material described above can also include a secondpolymer. The UHMWPE is mixed with a high density (HD) polyethylene toproduce a polyolefin mixture, where the polyolefin mixture has at least50% UHMWPE by weight. The filler used with this polyolefin mixture is ina range of from about 1:9 to about 15:1 filler to polyolefin mixture byweight. The resulting matrix comprises (or consists essentially of)UHMWPE and HD polyethylene and the particulate filler distributedthroughout the matrix. This microporous material has a machine direction(MD) tensile strength of greater than 25 N/mm².

A process for improving the wet out time of an uncoated microporousmembrane was developed comprising the steps of: providing an ultrahighmolecular weight polyethylene (UHMWPE); providing a particulate silicafiller; providing a processing plasticizer where said processingplasticizer may be a liquid at room temperature. Then mixing UHMWPE,filler and processing plasticizer together to form a mixture, having aweight ratio of filler to UHMWPE of from 1:9 to 15:1 by weight. Themixture is then extruded to form a sheet. The sheet is then processed,where processing is selected from the group consisting of: calendering,casting or blowing. The processed sheet then undergoes an extractionstep where all or part of the processing plasticizer is extracted fromthe sheet to produce a microporous matrix sheet which comprises UHMWPEand the particulate filler. In this matrix the filler is distributedthroughout the matrix. The macroporous matrix sheet is then calenderedto produce a microporous membrane with a reduction in thickness of atleast 5%. The resulting microporous membrane typically exhibits areduction in wet out time of 50% or more over said microporous matrixsheet without the use of any chemical surface coating treatments.

Additionally a process for improving the wet out time of an uncoatedmicroporous membrane comprising the steps of: providing an ultrahighmolecular weight polyethylene (UHMWPE); providing a particulate silicafiller; providing a processing plasticizer where said processingplasticizer may be a liquid at room temperature. Then mixing UHMWPE,filler and processing plasticizer together to form a mixture, having aweight ratio of filler to UHMWPE of from 1:9 to 15:1 by weight. Themixture is then extruded to form a sheet. The sheet is then processed,where processing is selected from the group consisting of: calendering,casting or blowing. The processed sheet then undergoes an extractionstep where all or part of the processing plasticizer is extracted fromthe sheet to produce a microporous matrix sheet which comprises UHMWPEand the particulate filler.

In this matrix the filler is distributed throughout the matrix. Themicroporous matrix sheet is then stretched in at least one stretchingdirection to a stretch ratio of at least about 1.5 to produce astretched microporous matrix sheet. This stretched microporous matrixsheet is then calendered to produce a microporous membrane with areduction in thickness of at least 5%. Where the resulting microporousmembrane, typically exhibits a reduction in wet out time of 50% or moreover the microporous matrix sheet without the use of any chemicalsurface coating treatments.

TEST PROCEDURES

-   Thickness—The membrane thickness values are reported in units of    micrometers (μm) and were measured using ASTM D374.-   Puncture Strength—The units of puncture strength are newtons and the    test procedure was ASTM D3763.-   Tensile Strength—Tensile strength was measured using ASTM D882 and    the units are N/mm2.-   Electrical Resistance—The units of electrical resistance are    mohm-cm₂.-   Shrink Testing—Both MD and TD shrink values were measured using a    modified version of ASTM D4802. The samples were cut into 5 inch    (12.7 cm) squares and put into an oven for 10 minutes at 100° C. The    units are percentage of change from the original dimension.-   Basis Weight—Basis weight was determined using ASTM D3776 and the    units are grams per square meter.-   Hg porosity—This was measured using Hg intrusion porosimetry.-   Gurley—The units are sec/10 cc and were measured by TAPPI T536    method.-   Wetout Time—A visual technique whereas a sample is gently placed    (not immersed) on the surface of water, and the time (in seconds) it    takes for the membrane to begin to darken is called the wetout time.-   Equipment—Calendering rolls used in these tests were stack rolls    having a diameter of 8 inches or 20.3 centimeters.

As can be seen from the Examples which follow, the resulting stretchedthen calendered microporous material exhibits improved dimensionalstability properties over a membrane which has only been stretched.

EXAMPLES

Example A is membrane containing the following:

Ratio Polymer Filler Filler- Example A UHMWPE SiO2 Plasticizer MinorsPolymer Extrusion  9.6% 25.0% 64.0% 1.4% 2.6 Extraction 23.5% 61.1%12.0% 1.4% 2.6

Now taking the material from Example A additional samples were preparedusing tenter frame equipment. This equipment allows for both uniaxialand biaxial stretching. The following parameters were used to producethese samples:

TABLE 1 Stretched Membrane Characteristics Modulus- Tensile- Net BackwebPuncture MD MD Sample # Stretch % (μm) (N) (MPa) (N/mm²) Elongation MD %A-10 300 173 6.7 71.2 11.9 23 A-11 300 173 8.3 69.6 14.1 27 A-12 400 1479.9 170.8 29.9 21 A-13 400 144 7.9 96.1 18.5 22 A-14 500 124 7.4 261.134.3 17 A-15 500 120 7.3 146.3 24.0 19 A-16 300 159 9.9 101.8 23.3 40A-17 400 150 11.4 149.5 31.3 28 A-18 300 × 350 80 3.3 23.5 4.6 21 A-19200 × 350 106 5.7 27.3 11.2 54A-18 and A-19 samples were biaxially stretched and produced using asequential stretching device. The other samples refer to stretchedmembranes in the MD direction (uniaxial) only.

TABLE 2 Stretched Membrane Characteristics (Continued) Modulus -Tensile - Elongation - Shrinkage - Shrinkage - TD TD TD MD TD Basis wtGurley Sample # (MPa) (N/mm²) % % % (gsm) (sec/100 cc) A-10 10.6 3.7 204−3.9 <1 59.5 58.8 A-11 8.6 3.9 229 −3.9 <1 58.7 56.0 A-12 7.1 3.7 303−11 <1 45.8 119.6 A-13 8.8 3.4 205 −3 <1 47.5 59.4 A-14 6.3 2.8 244 −7<1 38.3 90.2 A-15 7.4 3.2 219 −2.3 <1 41.7 53.4 A-16 11.6 3.9 234 −10.7−0.2 55.8 96.3 A-17 8.7 3.4 230 −12.3 −0.3 47.6 86.7 A-18 21.3 6.2 35−32 −44 15.6 15.4 A-19 19.6 8.3 54 −37 −44 20.5 36.8

TABLE 3 Stretched/Compressed Membrane Conditions and CharacteristicsCalender Tensile Tensile Gap Calender Temp Thickness MD TD Sample # μmPressure ° C. μm Puncture N N/mm2 N/mm2 A-16-F 0 Full 110 53.3 9.1 72.912.1 A-17-F 0 Full 110 48.3 10.4 86.0 11.5 A-16-M 25 Moderate 110 76.29.6 52.6 8.8 A-17-M 25 Moderate 110 71.1 10.5 77.4 10.5 A-16-S 100Slight 110 149.9 10.1 26.9 4.3 A-17-S 100 Slight 110 142.2 10.8 31.4 3.7A-16-F 0 Full 135 55.9 10.5 74.8 14.2 A-17-F 0 Full 135 53.3 11.1 90.114.2 A-16-M 25 Moderate 135 86.4 10.1 26.3 5.4 A-17-M 25 Moderate 13578.7 10.9 39.8 5.8 A-16-S 100 Slight 135 157.5 9.6 24.7 4.2 A-17-S 100Slight 135 149.9 10.8 28.3 4.2 A-18-F 0 Full 121 17.8 4.3 41.6 42.0A-18-M 20 Moderate 121 22.9 3.3 28.5 28.2 A-19-F 0 Full 121 20.3 8.264.9 54.9 A-19-M 20 Moderate 121 30.5 7.0 47.6 34.0

TABLE 4 Stretched/Compressed Membrane Characteristics (Continued)Elonga- Elonga- Wetout tion - tion - Shrinkage - Shrinkage - Time Sample# MD % TD % MD % TD % sec A-16-F 47 195 −0.5 0.9 18.0 A-17-F 34 193 −0.51.0 18.0 A-16-M 52 222 −2.6 0.1 41.0 A-17-M 39 244 −1.4 0.3 42.0 A-16-S46 208 −6.5 −0.2 115.0 A-17-S 31 242 −7.0 −0.3 160.0 A-16-F 51 188 −0.10.9 20.5 A-17-F 36 221 −0.4 0.8 18.5 A-16-M 46 207 −0.9 0.1 52.0 A-17-M36 235 −1.3 −0.1 46.5 A-16-S 56 208 −6.5 −0.2 79.5 A-17-S 39 261 −7.0−0.3 41.0 A-18-F 26 35 −0.8 0.0 4.5 A-18-M 24 50 −3.2 −4.6 9.0 A-19-F 5047 0.2 0.1 2.5 A-19-M 52 67 −2.4 −1.6 9.0Wetout times for uncalendered A-16, A-17, A-18 and A-19 were notobtained and found to be much greater than 10 minutes.

From the data presented in these Tables, several advantages can be seenwith the present process compared to the prior art. First, the stretchedand then calendered films have greatly improved dimensional stability,even at elevated temperatures. The thickness which can be achieved bythe stretching process alone is limited and thinner membranes can beachieved by calendering the membrane after stretching. Physical strengthis much improved after the calendering of a stretched microporousmaterial. Finally, the calendering process reduces the pore size andvarious degrees of calendering can be used to adjust to the desired poresize.

The present invention may be embodied in other forms without departingfrom the spirit and the essential attributes thereof, and, accordingly,reference should be made to the appended claims, rather than to theforegoing specification, as indicated the scope of the invention.

What is claimed is:
 1. A method for producing a battery with a batteryseparator, the battery separator is a microporous material, comprisingthe steps of: making the battery separator by mixing ultra highmolecular weight polyethylene (UHMWPE), siliceous filler and processingplasticizer together to form a mixture, having a weight ratio ofsiliceous filler to UHMWPE of from 1:9 to 15:1 by weight; wherein thesiliceous filler constitutes from about 5 percent to about 95 percent byweight of the microporous material and wherein the ratio of siliceousfiller to processing plasticizer is 1:15 to 3:1 by weight; extrudingsaid mixture to form a sheet; calendering said sheet; extracting all orpart of said processing plasticizer from said sheet to produce a matrixcomprising UHMWPE and said particulate siliceous filler, the siliceousfiller being distributed throughout said matrix, to produce amicroporous matrix sheet; stretching said microporous matrix sheet in atleast one stretching direction to a stretch ratio of at least about 1.5to produce a stretched microporous matrix sheet; and calendering saidstretched microporous matrix sheet under conditions to form pores havinga diameter from 0.01 micrometers to 0.1 micrometers, and placing thebattery separator in the battery.
 2. The method of claim 1 wherein thesiliceous filler constitutes from about 45 percent to about 95 percentby weight of the microporous material.
 3. The method of claim 1 whereinthe siliceous filler constitutes from about 55 percent to about 80percent by weight of the microporous material.
 4. The method of claim 1wherein the siliceous filler constitutes from about 61.1 percent toabout 80 percent by weight of the microporous material.
 5. The method ofclaim 1 wherein the siliceous filler is selected from silica,precipitated silica, silica gel, or fumed silica.
 6. The method of claim1 wherein the siliceous filler is silica.
 7. The method of claim 1wherein the ratio of siliceous filler to UHMWPE is about 2.6.
 8. Themethod of claim 1 where said microporous matrix sheet is stretched in atleast one direction to a stretch ratio from about 1.5 to
 15. 9. Themethod of claim 1 where said UHMWPE is mixed with a high density (HD)polyethylene to produce a polyolefin mixture, where said polyolefinmixture has at least 50% UHMWPE by weight; where said siliceous fillerto said polyolefin mixture is in a range of from 1:9 to 15:1 siliceousfiller to polyolefin mixture by weight and where said matrix comprisesUHMWPE and HD polyethylene and said particulate siliceous fillerdistributed throughout said matrix.
 10. A method for producing a batterywith a battery separator, the battery separator is a microporousmaterial, comprising the steps of: making the battery separator bymixing ultra high molecular weight polyethylene (UHMWPE), siliceousfiller and processing plasticizer together to form a mixture, having aweight ratio of siliceous filler to UHMWPE of from 1:9 to 15:1 byweight; wherein the siliceous filler constitutes from about 5 percent toabout 95 percent by weight of the microporous material; extruding saidmixture to form a sheet; calendering said sheet; extracting all or partof said processing plasticizer from said sheet to produce a matrixcomprising UHMWPE and said particulate siliceous filler, the siliceousfiller being distributed throughout said matrix, to produce amicroporous matrix sheet; stretching said microporous matrix sheet in atleast one stretching direction to a stretch ratio of at least about 1.5to produce a stretched microporous matrix sheet; and calendering saidstretched microporous matrix sheet under conditions so that theresulting microporous matrix sheet pores with a diameter from 0.01micrometers to 0.1 micrometers, and placing the battery separator in thebattery.
 11. The method of claim 10 wherein the siliceous fillerconstitutes from about 45 percent to about 95 percent by weight of themicroporous material.
 12. The method of claim 10 wherein the siliceousfiller constitutes from about 55 percent to about 80 percent by weightof the microporous material.
 13. The method of claim 10 wherein thesiliceous filler constitutes from about 61.1 percent to about 80 percentby weight of the microporous material.
 14. The method of claim 10wherein the siliceous filler is selected from silica, precipitatedsilica, silica gel, or fumed silica.
 15. The method of claim 10 whereinthe siliceous filler is silica.
 16. The method of claim 10 wherein theratio of siliceous filler to UHMWPE is about 2.6.
 17. The method ofclaim 10 where said microporous matrix sheet is stretched in at leastone direction to a stretch ratio from about 1.5 to
 15. 18. The method ofclaim 10 where said UHMWPE is mixed with a high density (HD)polyethylene to produce a polyolefin mixture, where said polyolefinmixture has at least 50% UHMWPE by weight; where said siliceous fillerto said polyolefin mixture is in a range of from 1:9 to 15:1 siliceousfiller to polyolefin mixture by weight and where said matrix comprisesUHMWPE and HD polyethylene and said particulate siliceous fillerdistributed throughout said matrix.
 19. The method of claim 1 wherein inaddition to the siliceous filler, a particulate water-insoluble filleris also present.
 20. The method of claim 19 wherein the particulatewater-insoluble filler is one or more selected from carbon black,activated carbon, carbon fibers, charcoal, graphite, titanium oxide,iron oxide, copper oxide, zinc oxide, lead oxide, tungsten, antimonyoxide, zirconia, magnesia, alumina, molybdenum disulfide, zinc sulfide,barium sulfate, strontium sulfate, calcium carbonate, and magnesiumcarbonate.
 21. The method of claim 20 wherein the particulatewater-insoluble filler includes carbon material.
 22. The method of claim21 wherein the carbon material is at least one of carbon black,activated carbons, and carbon fibers.
 23. The method of claim 1 whereinthe battery separator is a lead acid battery separator, and the batteryseparator is placed in a lead acid battery.
 24. The method of claim 10wherein the battery separator is a lead acid battery separator, and thebattery separator is placed in a lead acid battery.
 25. The method ofclaim 10, wherein in addition to the siliceous filler, a particulatewater-insoluble filler is also present.